In a typical configuration, an analytical balance presents itself as a unit which can be set up for operation on a work table, with a weighing pan inside an enclosed, transparent weighing compartment, a display panel and control elements in front, i.e. facing a human operator, and with a housing enclosure adjoining a rear side of the weighing compartment and containing mechanical, electrical and electronic operating parts of the balance.
In balances with five and more decimal digits displayed, the air temperature inside the weighing compartment is a critical factor, particularly in micro- and ultra-microbalances. If the temperature inside the weighing compartment is different from the ambient atmosphere, opening a weighing compartment door will give rise to strong air currents due to the temperature gradient between the inside and the outside. As it takes considerable time for the turbulence to settle, the weighing process will be slowed down by a long transient period before a stable result is displayed, and reproducibility will be negatively affected. After the door of the weighing compartment has been closed, air that has moved into the weighing compartment from the ambient atmosphere is heated up due to power dissipation of the electrical and electronic elements of the balance. As a result of the temperature rise, the air density (and consequently the air buoyancy effect on an object being weighed) will change over time, so that the indicated weighing result may drift (i.e. slowly change) over several minutes.
Furthermore, the problems caused by power dissipation of the electrical and electronic elements of balances increase with the growing demand for power-consuming features in balances, such as graphic user interfaces, network connections and connectivity to peripheral equipment.
According to a state-of-the-art solution which is used in micro- and ultra-microbalances manufactured by the assignee of the present invention, the temperature-sensitive parts of the balance are separated from the power-dissipating parts by incorporating them in different modular units that are tied to each other by cables and/or plug connectors. A first unit, the weighing module, includes the weighing compartment with the weighing pan and—directly adjoining and forming the rear wall of the weighing compartment—a first enclosure containing the weighing cell and only a required minimum of electronic components. A second unit, the electronic module, is housed in a second enclosure and essentially includes the analog and digital electronics and associated power supply circuits to support the weighing module. A third unit, the display module or user interface module, occupies a third enclosure and includes a touch screen display with associated electronics and power supply circuitry. This modular system provides a satisfactory solution to the aforementioned problem of temperature-induced air flows affecting the speed and accuracy of the weighing operation. However, it is inherently more expensive and more space-consuming than a balance that is configured as a single unit.
According to another solution developed by the assignee of the present invention and disclosed in U.S. Pat. No. 6,951,989, a balance which is configured as a single unit, with a weighing compartment and with a directly adjoining enclosure containing the weighing cell and electronics of the balance, includes a thermoelectric heat pump module, for example a Peltier element, which is arranged on the balance at a location outside of the weighing compartment. The cooling side of the thermoelectric heat pump module is thermally connected to the bottom of a heat-conductive vertical separating wall between the weighing compartment and the weighing cell compartment. Thus, with an appropriately selected, factory-set power level of the thermoelectric heat pump module, the interior of the weighing compartment can be held at a temperature level close to ambient, while a temperature gradient establishes itself wherein the temperature increases from the bottom to the top of the rear wall. This has the effect that the air temperature inside the weighing compartment likewise increases from the bottom to the top, which promotes a stable thermal stratification or layering of the air in the weighing compartment. Due to the resulting absence of convective air currents acting on the weighing pan, the indicated weighing result remains stable. This solution addresses the problem of air currents inside the weighing compartment, but it does not adequately mitigate the thermal influence on the weighing cell from the electronics and power supply which share the same enclosure with the weighing cell.
In another analytical balance of single-unit configuration, which is described in DE 10 2009 055 622 A1, the weighing compartment with the weighing pan sits on top of an enclosed base containing the weighing cell and electronics. A first heat-conducting plate which forms a floor of the weighing compartment is connected to the cooling side of a thermoelectric heat pump module, while the heating side of the thermoelectric heat pump module is connected by way of a heat pipe to a second heat-conducting plate forming a ceiling of the weighing compartment. It is proposed to regulate the power of the thermoelectric heat pump module by way of a feedback control circuit with a first and a second temperature sensor arranged respectively, near the top and the bottom of the interior of the weighing compartment, and wherein the average of the signals of the first and the second temperature sensor is used as the feedback quantity. In a further developed embodiment, a third temperature sensor is added outside the weighing compartment, i.e. exposed to the ambient atmosphere, and the power of the thermoelectric heat pump module is regulated dependent on the aforementioned average signal of the first and the second temperature sensor and additionally dependent on the signal of the third temperature sensor.
In the foregoing examples of state-of-the-art solutions to prevent convective air currents in the weighing compartment of an analytical balance which occur as a result of the heat generated by the electronics, one will recognize a progression from:
a) an entirely passive solution that relies on housing the power-dissipating parts of the balance in spatially separated modular units, to
b) an active, open-loop solution using a thermoelectric heat pump module that operates without feedback control, to
c) an active, closed-loop solution using a thermoelectric heat pump module whose power is regulated by feedback control based on one or more actual temperatures that are measured by temperature sensors.
It is therefore the objective of the present invention to provide a balance, specifically an analytical balance having a resolution of 10−5 to 10−7 grams, with active, feedback-regulated control means to prevent heat that originates from the electronics and power supply circuits of the balance from entering into the weighing cell and the weighing compartment.